Since a plasma display panel (hereinafter, referred to as a “PDP”) can realize a high definition and a large screen, 65-inch class televisions are commercialized. Recently, PDPs have been applied to high-definition television in which the number of scan lines is twice or more than that of a conventional NTSC method. Meanwhile, from the viewpoint of environmental problems, PDPs without containing a lead component have been demanded.
A PDP basically includes a front panel and a rear panel. The front panel includes a glass substrate of sodium borosilicate glass produced by a float process; display electrodes each composed of striped transparent electrode and bus electrode formed on one principal surface of the glass substrate; a dielectric layer covering the display electrodes and functioning as a capacitor; and a protective layer made of magnesium oxide (MgO) formed on the dielectric layer. On the other hand, the rear panel includes a glass substrate; striped address electrodes formed on one principal surface of the glass substrate; a base dielectric layer covering the address electrodes; barrier ribs formed on the base dielectric layer; and phosphor layers formed between the barrier ribs and emitting red, green and blue light, respectively.
The front panel and the rear panel are hermetically sealed so that the surfaces having electrodes face each other. Discharge gas of Ne—Xe is filled in discharge space partitioned by the barrier ribs at a pressure of 400 Torr to 600 Torr. The PDP realizes a color image display by selectively applying a video signal voltage to the display electrode so as to generate electric discharge, thus exciting a phosphor layer of each color with ultraviolet ray generated by the electric discharge so as to emit red, green and blue light.
In such PDPs, the role of the protective layer formed on the dielectric layer of the front panel includes protecting the dielectric layer from ion bombardment by discharge, emitting initial electrons so as to generate address discharge, and the like. Protecting the dielectric layer from ion bombardment is an important role for preventing a discharge voltage from increasing. Emitting initial electrons so as to generate address discharge is an important role for preventing address discharge error that may cause flicker of an image.
In order to reduce flicker of an image by increasing the number of initial electrons emitted from the protective layer, an example in which an impurity is added to MgO and an example in which MgO particles are formed on an MgO protective layer are disclosed for instance (see, for example, Patent Documents 1, 2 and 3).
Recently, televisions have realized higher definition. In the market, low cost, low power consumption and high brightness full HD (high definition) (1920×1080 pixels: progressive display) PDPs have been demanded. Since an electron emission property from a protective layer determines an image quality of a PDP, it is very important to control the electron emission property.
An attempt to improve the electron emission property has been made by mixing an impurity in a protective layer. However, when the electron emission property is improved by mixing an impurity in the protective layer, electric charges are accumulated on the surface of the protective layer, thus increasing a damping factor, that is, reducing electric charges to be used as a memory function over time. Therefore, in order to suppress this it is necessary to take measures, for example, an applied voltage needs to be increased. Thus, a protective layer should have two conflicting properties, a high electron emission property and a high electric charge retention property, that is, a property of reducing a damping factor of electric charge as a memory function.    [Patent document 1] Japanese Patent Unexamined Publication No. 2002-260535    [Patent document 2] Japanese Patent Unexamined Publication No. H11-339665    [Patent document 1] Japanese Patent Unexamined Publication No. 2006-59779